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<h2>ReusableConstraint (NUnit 2.5.6)</h2>

<p>Normally constraints just work. However, attempting to reuse the 
same constraint in several places can lead to unexpected results.

<p>Consider the following code as an example:

<code><pre>
    Constraint myConstraint = Is.Not.Null;
    Assert.That("not a null", myConstraint); // Passes, of course
    Assert.That("not a null", myConstraint); // Fails! What's that about?
</pre></code>

<p>We'll save the technical explanation for later and show the
solution first:

<code><pre>
    ReusableConstraint myConstraint = Is.Not.Null;
    Assert.That("not a null", myConstraint); // Passes
    Assert.That("not a null", myConstraint); // Passes
</pre></code>

Or alternatively..

<code><pre>
    var myConstraint = new ReusableConstraint(Is.Not.Null);
    Assert.That("not a null", myConstraint); // Passes
    Assert.That("not a null", myConstraint); // Passes
</pre></code>

<h3>Technical Explanation</h3>

<p>In the original example, the value assigned to myConstraint is
known as an <b>unresolved</b> constraint. In fact, it's an
unresolved NullConstraint, because that was the last constraint 
encountered in the expression. It's associated with a <b>Not</b>
operator that has not yet been applied.

<p>That's OK for use with Assert.That(), because the method
knows how to resolve a constraint before using it. Assert.That()
resolves this constraint to a NotConstraint referencing the
original NullConstraint.

<p>Of course, the original reference in myConstraint is left
unchanged in all of this. But the EqualConstraint it points
to has now been resolved. It is now a <b>resolved</b> constraint
and can't be resolved again by the second Assert.That(), which
only sees the NullConstraint and not the NotConstraint.

<p>So, for reusability, what we want to save is the result
of resolving the constraint, in this case

<pre>    NotConstraint => NullConstraint</pre>

That's what <b>ReusableConstraint</b> does for us. It resolves
the full expression and saves the result. Then it passes all
operations on to that saved result.

<h3>When to Use It</h3>

<p>Use this constraint any time you want to reuse a constraint
expression and you'll be safe.

<p>If you like to take chances, you'll find that you can
avoid using it in the following cases...

<ol>
<li> With a simple constraint involving no operators, like...

<pre>
    Constraint myConstraint = Is.Null;
    Constraint myConstraint = Is.EqualTo(42);
</pre>

<li> With any constraint you construct using new, without
using the "dotted" constraint syntax...

<pre>
    Constraint myConstraint = new NotConstraint(new NullConstraint());
    Constraint myConstraint = new AndConstraint(
        new GreaterThanConstraint(0), 
        new LessThanConstraint(100));
</pre>

<p>However, there is no significant penalty to using <b>ReusableConstraint</b>.
It makes your intent much clearer and the exceptions listed are accidents of
the internal implementation and could disappear in future releases.


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